1. Field of the Invention
The present invention relates to a bearing cap structure for an engine, and more particularly to a bearing cap structure for an engine in which a core member (preform) formed of a different material from a base material is cast therein.
2. Description of the Related Art
In an engine for a vehicle, the upper part of a cylinder block to be an engine member is provided with a cylinder head. Further, the lower part of the cylinder block is provided with a crank lower case to be a bearing cap including a lower case side journal portion to be a cap side journal portion which pivotally supports a crankshaft to be a shaft member in cooperation with a block side journal portion of the cylinder block.
More specifically, as shown in FIG. 6, a crank lower case 106 attached to the lower part of a cylinder block 104 of an engine 102 is formed of aluminum and is molded by die casting, for example, and includes a bonding portion 108 to be bonded to the lower surface of the cylinder block 104, a semicircular lower case side journal portion 110 for opening upward to pivotally support a crankshaft (not shown), lower case bolt holes 112-1 and 112-2 oriented in a vertical direction on both sides of the lower case side journal portion 110, and lower case outside walls 114-1 and 114-2 on both end sides.
Moreover, such a bearing cap structure for an engine has been disclosed in JP-A-2000-205037, for example. This publication has described that left and right outside walls are coupled to each other, a preform (a core member) formed of a reinforced fiber and traversing the left and right outside walls is cast into a bulkhead (a partition) constituting a bearing portion of a crankshaft and the preform is impregnated with dissolved molten metal to constitute a cylinder block.
In the bearing cap structure for an engine, conventionally, a cylinder block to be an engine member and a crank lower case to be a bearing cap are formed of aluminum in order to reduce the weight of the engine in many cases. The cylinder block and the crank lower case which are formed of aluminum have higher coefficients of linear expansion than those of a cylinder block and a crank lower case which are formed of cast iron. Therefore, a clearance of a crank journal portion in a high temperature region tends to be increased in order to obtain a clearance between a crankshaft and the crank journal portion in a low temperature region.
On the other hand, there is a method of casting a different material from an aluminum material (a base material) into an inner part when casting a crank lower case formed of aluminum in order to reduce the coefficient of linear expansion of an engine member. In the method of casting the different material as shown in FIG. 7, a core member (a preform) 116 having a predetermined shape which is formed of a fibrous material is cast into a casting mold (not shown) to change the lower case side journal portion 110 into an alumina alloy, thereby reducing the coefficient of linear expansion.
However, it is hard to fix and hold the core member 116 during the casting. When the base point of a radius of curvature R1 of an inner peripheral surface 110F of the lower case side journal portion 110 and a radius of curvature R2 of an inner peripheral surface 118F of a core member side journal portion 118 is set to be an identical lower case center O, a thickness (distance) T between the inner peripheral surface 110F of the lower case side journal portion 110 and the inner peripheral surface 118F of the core member side journal portion 118 is taken into consideration. Accordingly, the radius of curvature R2 of the inner peripheral surface 118F of the core member side journal portion 118 is made slightly greater than the radius of curvature R1 of the inner peripheral surface 110F of the lower case side journal portion 110 (R2=R1+T) In the case in which the casting is to be carried out while slightly leaving a portion formed of aluminum (a thickness T: constant) between the inner peripheral surface 110F of the lower case side journal portion 110 and the inner peripheral surface 118F of the core member side journal portion 118, the motion of the core member 116 in a vertical direction is controlled by using step portions 124-1U and 124-1B and step portions 124-2U and 124-2B as shown in FIGS. 8 and 9. The step portions 124-1U and 124-1B are provided on cast-off pins 122-1U and 122-1B for upper and lower parts in one side, and the cast-off pins 122-1U and 122-1B are inserted into a core member bolt hole 120-1 of the core member 116 in the one side. The step portions 124-2U and 124-2B are provided on cast-off pins 122-2U and 122-2B for upper and lower parts in the other side, and the cast-off pins 122-2U and 122-2B are inserted into a core member bolt hole 120-2 of the core member 116 in the other side.
As shown in FIG. 9, for example, the step portion 124-1U for the one-side upper part will be described. A clearance S is present between a hole end portion 120-1M on the upper side of the core member bolt hole 120-1 of the core member 116 and the outer peripheral surface of the cast-off pin 122-1U for the upper part. As shown in FIG. 10, therefore, a shift D-1 is present between the lower case bolt hole 112-1 and the cast-off pins 122-1U and 122-1B for the one-side upper and lower parts, and the other side shift D-2 is present between the lower case bolt hole 112-2 and the cast-off pins 122-2U and 122-2B for the other side upper and lower parts in a transverse direction. Here, the transverse direction is a molten metal flowing direction in which an aluminum material to be a base material flows. Accordingly, the core member 116 is shifted toward the side of the lower case side wall 114-2. More specifically, a core member center (a preform center) Q is therefore moved by a center shift E toward the side of the lower case side wall 114-2 with respect to the lower case center O (as shown in a one-dotted chain line of FIG. 10). In some cases, consequently, a part of the inner peripheral surface 118F of the core member side journal portion 118 is exposed to the inner peripheral surface 110F of the lower case side journal portion 110 (as shown in a preform exposed portion P of FIG. 10). In the case in which the core member side journal portion 118 is exposed to the lower case side journal portion 110, thus, the osmosis of the aluminum material from the exposed portion is eliminated. For this reason, there is a drawback that the lower case side journal portion 110 cannot be uniformly changed into an alumina alloy and a casting error is therefore made.
In order to eliminate the drawbacks, therefore, the present invention provides a bearing cap structure for an engine in which there is provided a bearing cap including a bonding portion to be bonded to an engine member and a cap side journal portion for pivotally supporting a shaft member in cooperation with a member side journal portion of the engine member, a core member formed of a different material from a base material is provided in a casting mold when casting the bearing cap. The base material is injected from a pouring gate to cast the core member therein. The core member has a core member side journal portion provided along the cap side journal portion and a core member bolt hole on both sides of the core member side journal portion. Further, the core member is provided in a separation state from a surface of the casting mold through a cast-off pin including a step portion in a vertical direction in the casting mold. A radius of curvature of the core member side journal portion of the core member is made greater than a radius of curvature of the cap side journal portion of the bearing cap in such a manner that a clearance between the cap side journal portion and the core member side journal portion is larger than a clearance between the core member bolt hole and the cast-off pin. The pouring gate is provided on a side surface of the casting mold such that the base material is injected along the bonding portion of the bearing cap.